Rebel fighters launched an assault in Syria's divided northern city of Aleppo on Tuesday and fired rockets on a hospital in the latest violence to hit civilians as diplomats struggled to restore an unravelling ceasefire and resurrect peace talks.

The Syrian Observatory for Human Rights, a British-based group that monitors the conflict, said rebel rockets had killed 19 people in government-held territory, including an unspecified number at the al-Dabit hospital.

It described a rebel offensive that led to casualties on both sides. Rebels had hit a government gun position with a guided missile.

The Syrian state-run Ikhbariya news channel said three women were killed at the hospital and 17 people wounded. A Damascus Information Ministry statement called it a crime against humanity.

The rebel attack followed government air strikes on rebel areas, including one that hit a hospital last week, which medical charity Medecins Sans Frontieres said killed 55 civilians.

Aleppo has been the scene of the worst surge in fighting in recent days, wrecking the first major ceasefire of the five-year-old civil war, sponsored by the United States and Russia, which had held since February.

The United States and Russia have taken the leading roles in diplomacy since Moscow joined the war last year with an air campaign that tipped the balance of power in favor of President Bashar al-Assad, its ally.

In an effort to revive the ceasefire, temporary local truces have been put in place in two parts of Syria, but those have not been extended to Aleppo, Syria's largest city before the war and its biggest strategic prize now.

U.N. Syria envoy Staffan de Mistura, who met U.S. Secretary of State John Kerry in Geneva on Monday and Russian Foreign Minister Sergei Lavrov in Moscow on Tuesday, said he hoped the truce could be extended to Aleppo swiftly.

"We all hope that ... in a few hours we can relaunch the cessation of hostilities. If we can do this, we will be back on the right track," de Mistura said. If the truce were extended to Aleppo, peace talks could resume, he said.

Lavrov said, "The process of agreeing a ceasefire in Aleppo is being finished right now between Russian and American military personnel."

He added that he hoped it could be announced in the near future, "maybe even in the coming hours."

Kerry warns Assad of 'repercussions'

In pointed remarks in Washington on Tuesday, Kerry warned Assad of "repercussions" if he did not stick to the ceasefire and move forward with a political transition aimed at ending the war in Syria.

"If Assad does not adhere to this, there will clearly be repercussions, and one of them may be the total destruction of the ceasefire and then go back to war," Kerry told reporters a day after the meetings in Geneva.

"I don't think Russia wants that. I don't think Assad is going to benefit from that. There may be even other repercussions being discussed," he said.

It was not clear what Kerry meant by repercussions. Obama administration officials have previously warned of consequences for Assad's actions in Syria's civil war, but critics have said the United States has failed to follow through with a more aggressive response.

Kerry said fighting in Aleppo could escalate if the ceasefire was not extended to the city.

The Aleppo fighting threatens to wreck the first peace talks involving the warring parties, which are due to resume at an unspecified date after breaking up in April when the opposition delegation walked out.

Washington and Moscow have announced a new joint centre in Geneva to monitor the ceasefire, to be staffed around the clock by U.S. and Russian officers.

The two governments have been working to extend the local truces to Aleppo but so far have not been able to do so, although both have expressed optimism that this can be achieved.

Big prize

Aleppo remains the biggest prize for Assad's forces hoping to take full control of the city. The nearby countryside includes the last strip of the Syria-Turkish border in the hands of Sunni rebels.

The Observatory has reported 279 civilians killed on both sides in Aleppo since April 22, by intense government air strikes and rebel shelling, with 155 killed in opposition-held areas and 124 in government-held districts.

Tuesday's rebel ground assault focused on the Jamiat al-Zahraa area of the city, where insurgent groups took a few buildings before advances were checked by the arrival of reinforcements on the government side, the Observatory said.

A Syrian army source said a car bomb was used in an attack nearby, but the assault had failed. "Matters had been moving towards Aleppo being included in the truce, but it seems there are those who do not want that," the source said.

A Syrian army statement on Tuesday said the rebel attack on the hospital came "at a time when international and local efforts are being made to shore up" the ceasefire and extend the truce to Aleppo.

In a statement, the Nour Al Edin Al Zenkey rebel group accused the government of launching the first ground assault on Tuesday, saying it had attacked west of Aleppo. The group said the assault was successfully repelled and was followed by a counterattack in which rebels captured several positions.

The civil war in Syria has killed hundred of thousands of people, driven millions from their homes, created the world's worst refugee crisis and provided a base for Islamic State militants who have launched attacks elsewhere.

All diplomatic efforts to resolve it have foundered over the fate of Assad, who refuses to accept opposition demands that he leave power.

Washington is among Western and regional powers that say Assad must leave office.

A man walks past the rubble of damaged buildings after an airstrike in the rebel held area of Aleppo's Baedeen district, Syria, May 3, 2016. Photo: Reuters/Abdalrhman Ismail